The present invention relates to a nonvolatile, integrated-circuit memory such as an electrically-erasable, electrically-programmable read-only-memory (EEPROM), and more particularly to an EEPROM memory cell that is programmed by hot-carrier injection in one region and is erased by Fowler-Nordheim tunneling in another region, and to a method of fabricating such a device.
EEPROMs employing single transistor memory cells, using hot-carrier injection for programing and Fowler-Nordheim tunneling for erasure have been described in: (a) "A Single Transistor EEPROM cell and its implementation in a 512K CMOS EEPROM", S. Mukherjee et al., IEDM 1985 (p. 616-619); and (b) "A 90ns 100K Erase/Program Cycle Megabit Flash Memory", V. Kynett et al., ISSCC 1989 (p. 140-141). References (a) is also discussed in U.S. Pat. No. 4,698,787.
Referring to FIG. 1a, in prior-art memory cells, such as those discussed in references (a) and (b), the field oxide isolation regions 25 are used to improved capacitive coupling between control gate 14 (poly 2) and floating gate 13 (poly 1), by increasing the capacitance area between control gate and floating gate in addition to providing isolation between adjacent cells. The source line 17 (diffused N+) runs substantially parallel to the wordlines 15 and separates the field oxide regions 25 between pairs of wordlines 15. Drain regions 12 are shared between each pair of wordlines 15. The drains 12 in a column are connected by a metal column line (not shown). Each drain contact with the metal line is isolated from the adjacent contact by the field oxide regions 25.
Thus, the field oxide regions 25 provide both improved control-gate/floating-gate capacitive coupling as well as isolation between adjacent cells. The presence of field oxide regions 25 separated by a continuous source line 17 avoids the need for a metal line contacting sources 11, saving valuable silicon real estate. These field oxide regions 25 are designed and formed using masks having straight lines. However, after a LOCOS process, the field oxide regions 25 look like "dog bones" (as shown by FIGS. 1a-1b) for at least two reasons. The first reason is related to lithographic rounding at the corners and the second reason is related to the field oxide growth rate differences between narrow regions and wide regions.
The dog-bone-shaped field oxide regions 25 result in undesirable effects. Because of manufacturing misalignments, such as that illustrated by the different sizes of regions A and B in FIG. 1b, adjacent cells sharing the same source line 17 may have different coupling coefficients between control-gates 14 and source regions 11, resulting in bimodal distributions of array programming, reading and erasing characteristics. In addition, the cell width is also nonuniform because of the "dog bone" effect.
Accordingly, a need exists for a cell array structure that avoids these weaknesses.